GAZA CITY — An Israeli airstrike tore through a crowded shopping area Wednesday shortly after tank shells struck a United Nations-run school used by Gazans seeking refuge, officials said, as the violent conflict between Israel and Hamas militants reached its 23rd day.

Gaza Health Ministry official Ashraf al-Kidra said 16 people were killed and more than 150 wounded at the shopping area that was busy because residents thought a cease-fire was in place.

Israel had announced a four-hour humanitarian cease-fire in some areas of Gaza shortly after the school explosion. Al-Kidra said the school blast killed at least 17 people and wounded about 90. The Israeli military said mortar shells had been fired from near the school, and that its soldiers fired back.

The school explosion came hours after United Nations Relief and Works Agency reported it had determined that weapons had been stored in one of its Gaza schools. It's the third such discovery in two weeks. UNRWA condemned the use of the school for stashing weaponry, but added that the school was closed for the summer and was not being used as a shelter.

White House National Security Council spokeswoman Bernadette Meehan condemned the shelling of the school and the use of schools to stash weapons. Israel claims that Hamas militants routinely use infrastructure such as mosques and schools to conceal weapons. Hamas says that Israel's military makes indiscriminate use of its overwhelming military power.

The Israel Defense Forces (IDF) had no immediate comment on the shopping center attack, but did accuse Hamas of firing at least 26 rockets during the four-hour window.

IDF also said three Israeli soldiers were killed "during operational activity" in Gaza. Earlier, the Israeli military had said an "elite" IDF unit had uncovered a tunnel opening inside a house in the southern Gaza Strip. IDF said two explosive devices were detonated by terrorists, causing the residence to collapse and wounding several soldiers.

Israeli Prime Minister Benjamin Netanyahu has said Israel won't stop its offensive until all tunnels providing access into Israel have been destroyed. Polls show the Israeli offensive continues to hold wide support among Israelis.

"They want to kill us. We have no choice," said Jerusalem resident Gil Yair, 39, referring to Hamas, the militant group that controls Gaza. "They are holding a gun to our head and we have to take control of the situation."

Almost 60 Israelis, the vast majority soldiers, have been killed since fighting broke out July 8, the IDF says. More than 1,300 Palestinians have been killed, Gaza's Health Ministry says. UNRWA says the number of displaced people taking refuge in its shelters is approaching 10% of Gaza's population, with approximately 182,604 in 82 schools.

Adnan Abu Hasna, a spokesman UNRWA, said the U.N. school in the Jebaliya refugee camp hit Wednesday contained hundreds of Palestinians displaced by the conflict who had sought shelter there from Israeli tank fire.

Fayez Abu Dayeh, the school's principal, said shells hit two classrooms and a bathroom. Palestinian health official Ashraf al-Kidra corroborated that account. Six U.N. schools have now taken direct hits during recent Israeli bombardments, according to UNRWA.

Explosions lit up the sky as Israeli forces hit the only power plant on the Gaza strip on Tuesday. Israeli Prime Minister Benjamin Netanyahu warned Gaza of a "prolonged" military campaign as the fighting continues in the region.

Diplomatic efforts to reach a cease-fire have been unsuccessful. Hamas has said it will not agree to a truce unless its demands are met, including the lifting of a blockade on Gaza.

"Lifting the siege and reopening border crossings is a basic human right," said Abu Ali, from Gaza City. "Everyone who lives in Gaza will tell you we have had enough. It's time allow the people of this prison, Gaza, to have a normal life."

"I didn't expect this war to drag on for the entire month," she said. "I wasn't expecting the ground troops to enter (Gaza) but I think it is essential to destroy the tunnels."

Palestinians celebrate the announcement of a cease-fire on Aug. 26 in Gaza City. Palestinian President Mahmoud Abbas announced an Egyptian-brokered cease-fire proposal to end seven weeks of fighting between Israel and militant groups in the Gaza Strip. (Photo: Mohammed Saber, European Pressphoto Agency)

A worker removes debris from the Cohen family bedroom after a missile fired by militants in the Gaza Strip made a direct hit in their home in Ashkelon, Israel. (Photo: Abir Sultan, European Pressphoto Agency)

A Palestinian boy walks across the rubble of a house belonging to the al-Dakhani family after an Israeli airstrike on the Rafah refugee camp, southern Gaza Strip. Two people were wounded in the attack. (Photo: Eyad Baba, AP)

A man looks out the window of a damaged classroom after a rocket, fired by Palestinian militants from the Gaza Strip, landed in the courtyard of a kindergarten in Ashdod, Israel. (Photo: Jack Guez, AFP/Getty Images)

A Palestinian man sweeps the floor of his home that was damaged after a mosque across the street was hit by an Israeli airstrike in Beit Lahia in the northern Gaza Strip. (Photo: Roberto Schmidt, AFP/Getty Images)

A Palestinian woman watches from a hole in a home belonging to Hamas financial official Mohammed al-Ghul after it was targeted by an Israeli airstrike in Gaza City. One of several targeted airstrikes by Israeli forces killed Al-Ghul in his vehicle. (Photo: Mohammed Abed, AFP/Getty Images)

Relatives and close family friends of Daniel Turgeman touch his coffin during the boy's funeral on Aug. 24 in Yevul, Israel. Turgeman, 4, was killed in a mortar attack fired by Palestinians inside the Gaza Strip on Aug. 22. (Photo: Jim Hollander, European Pressphoto Agency)

An Israeli firefighter extinguishes a fire after rockets fired by Palestinian militants from the Gaza Strip hit a field near the southern Israeli city of Sderot. (Photo: Menahem Kahana, AFP/Getty Images)

Palestinians carry the body of the wife of Mohammed Deif, Israel's most-wanted man, at the Jabaliya refugee camp in the northern Gaza Strip. According to media reports, Deif's wife and son were killed Aug. 20 in an Israeli airstrike on Gaza City. Israel holds Deif, the leader of the Qassam Brigades, Hamas' military wing, accountable for directing the Gaza conflict from underground. (Photo: Mohammed Saber, European Pressphoto Agency)

Israeli soldiers in an undisclosed southern Israeli location take cover inside a cement pipe near the border with the Gaza Strip as a 'red alert' sounds, signaling that Palestinian militants have fired a rocket toward the area. The Israeli army reported that Palestinians shot about 180 rockets and mortars since the collapse of the cease-fire, with scores of Israeli strikes inside the Gaza Strip. (Photo: Avi Roccah, European Pressphoto Agency)

Palestinians grieve over the death of Widad Mustafa Deif, 27, and her 8-month-old son, Ali Mohammed Deif, during their funeral in Jabaliya refugee camp. They were killed by an Israeli strike Aug. 20. (Photo: Khalil Hamra, AP)

A Palestinian boy spots two Israeli drones in the late afternoon Aug. 19. Palestinians fled their homes in neighborhoods of eastern Gaza City carrying bags of clothes, pillows and mattresses after renewed Israeli airstrikes, witnesses said. Nine days of relative quiet in the skies over Gaza came to an abrupt halt when rockets struck Israel just hours before the truce was to expire at midnight local time. Israel immediately ordered a military response, with warplanes striking targets across the coastal strip. (Photo: Roberto Schmidt, AFP/Getty Images)

A rocket fired by Palestinian militants inside the Gaza Strip rises into the night sky. More than 50 rockets were fired toward Israeli civilian centers from Gaza on Aug. 19, shattering the 24-hour cease-fire, and Israel responded with multiple air force attacks on Gaza, with deaths and many casualties reported. (Photo: Avi Roccah, European Pressphoto Agency)

Palestinian rescuers clear the rubble of a destroyed house following an Israeli airstrike in Gaza City. The Israeli airstrike killed a young girl and a woman, wounding 16 other people. (Photo: Mohammed Abed, AFP/Getty Images)

An Egyptian soldier stands guard on the Egyptian side of the border between Egypt and the Gaza Strip on Aug. 13, overlooking the damage in Rafah caused by a month of fighting between the Israeli military and Hamas militants. (Photo: Said Khatib, AFP/Getty Images)

Palestinian mourners carry the body of Zakariah al-Aqrah, 21, during his funeral in the West Bank village of Qabalan. The Israeli military said it killed al-Aqrah early Monday morning after he opened fire on an Israeli force that had come to arrest him in connection with shootings targeting Israeli soldiers two weeks ago. (Photo: Majdi Mohammed, AP)

A Palestinian man standing in a crowd of onlookers reacts to watching a soap factory go up in flames moments after it was hit by an Israeli airstrike in Gaza City, Aug, 10. (Photo: Roberto Schmidt, AFP/Getty Images)